Tomography is an imaging method in which cross-sectional images of a three-dimensional recording area are reconstructed. A tomography machine has a recording unit with a central system axis. The recording unit can be realized in the form of a ring or a tunnel. The recording unit furthermore has an isocenter in which the conditions for a tomographic recording are particularly advantageous. Typically, the system axis and the isocenter are caused to overlap at least partly. The recording area can be traversed along the system axis and therefore through the isocenter during the tomographic recording. At the end of the tomographic recording, the projections are processed such that a tomographic image is created.
In the case of X-ray tomography, X-ray projections are recorded at various projection angles. In this respect, the recording unit rotates together with a radiation source and also a radiation detector around the system axis and also around the recording area. The point of intersection between the beams emitted by the radiation source and the system axis forms the isocenter of the recording unit. In the case of magnetic resonance tomography, in particular, the radiation detector can be arranged in the form of local coils outside the recording unit. Furthermore, the system axis is arranged parallel to a main magnetic field in the case of magnetic resonance tomography, with the isocenter being characterized by a particularly homogeneous main magnetic field.
A decisive factor for the quality of a tomographic image of this type is how the recording area of the subject is positioned. As a rule it is desirable, for example, to position the radiological central point of the recording area, or an examination area situated within the recording area, in the isocenter of the recording unit of a tomography machine. As a result, attenuation of the radiation takes place as evenly as possible. Precise positioning is important in the clinical environment in particular if the recording area involves a bodily region of a patient. This is because a repeat tomographic recording due to incorrect positioning goes hand in hand with an additional dose exposure and also a considerable time delay in routine clinical activities. Furthermore, the highest possible quality of the tomographic image is indispensable in clinical diagnostics.
The positioning of the patient is traditionally carried out by an operator traversing the patient table manually. Furthermore, the operator uses an optical marking, which is projected on to the patient, typically in the form of a laser line, for the positioning. The patient's positioning perpendicular to the system axis is particularly problematic in this respect, in particular the positioning in the vertical direction. Due to the high level of time pressure in routine clinical activities, the patient's vertical position in particular is frequently set with insufficient precision.